The present invention relates to a method and apparatus for manufacturing a magnetic recording medium, and particularly relates to a method and apparatus for manufacturing a magnetic recording medium in which a thin metal film is formed through vacuum evaporation on a continuously conveyed flexible belt-like support.
Conventionally there have been known various kinds of thin film forming methods for forming a thin metal film of a single metal, an alloy, or the like, on a base material made of a macromolecular substance or the like to thereby produce a magnetic recording medium, a thin-film integrated circuit, an energy converting device, or the like. A vacuum evaporation method is effective for efficiently forming a thin metal film on a continuously conveyed flexible belt-like support (hereinafter referred to "a web").
For example, a coating-type manufacturing method in which a powder magnetic material is applied, together with an organic binder or the like on a nonmagnetic web and then dried to thereby form a magnetic layer has been widely used conventionally to manufacture magnetic recording media.
Recently, however, it has been strongly desired to improve the maximum possible recording density of magnetic recording media, and for this purpose it is necessary to improve the magnetic force of the magnetic recording medium and reduce the thickness of the magnetic layer.
Compared with a coating-type magnetic recording medium prepared by dispersing a magnetic coating liquid containing a powdered magnetic material in an organic binder and applying the coating liquid onto a nonmagnetic web and then drying the web, in a ferromagnetic thin metal film type magnetic recording medium in which the coercive force Hc and residual magnetic flux density Br are large, the magnetic layer can be made extremely thin, and a thin metal film of a ferromagnetic metal material can be formed directly on a nonmagnetic web.
Although wet-type methods such as electrolytic plating, nonelectrolytic plating, etc., and dry type methods such as vacuum evaporation, sputtering, ion plating, CVD, etc., have been used for manufacturing ferromagnetic metal thin-film type magnetic recording media, the vacuum evaporation method is generally considered the most suitable in view of the accompanying high film forming rate and productivity.
FIG. 4 schematically shows a conventional vacuum evaporation apparatus 20. The inside of a vacuum vessel 32 is kept, by an exhaust device 33, at a pressure in a range of 10.sup.-4 Torr to 10.sup.-6 Torr. A web 30 of a high polymer material, such as polyethylene terephthalate, is pulled from a supply roll 22 by a take-up roll 23, and between rolls 22 and 23 it is wound around the circumferential side surface of a cylindrical cooling can 24. The web 30 is arranged so as to be moved and conveyed in the direction indicated by an arrow A in synchronism with the rotation of the cooling can 24.
An evaporation source 21 is provided under the cooling can 24 so as to heat and fuse an evaporation material 28 in a crucible 27 by use of an electron beam 31 to thereby generate an evaporation stream 29. The evaporation stream 29 is applied to the web 30 to thereby form a thin film on the web 30.
In order to perform continuous evaporation on the web 30, the electron beam 31 generally heats the evaporation material 28 while scanning the web in the width direction of the web. Accordingly, the crucible 27 must have a width which is at least equal to the width of the vessel in the direction parallel to the width direction of the web.
Further, in the case where a ferromagnetic metal thin-film type magnetic recording medium is manufactured using such a vacuum evaporation apparatus 20, it has been known that magnetic characteristics such as the coercive force Hc and the square ratio SQ of the magnetic layer formed on the web 30 can be improved by cutting off, through masks 25, a portion of the evaporation stream 29. The evaporation source 21 is generally provided at a position removed from the central axis of the cooling can 24. In this method, termed a "slant-incidence" evaporation method, the evaporation stream 29 is made incident on the web 30 at an oblique angle. In the slant evaporation method, the growth shape of the evaporated particles of the evaporated layer formed on the web 30 is preferable in view of certain desired characteristics of the layer.
Conventionally, in order to form two magnetic layers using an apparatus as shown in FIG. 4, for example, it is necessary that, after a first evaporation step has been completed, to reload the original roll on the feeding side, and then perform a second evaporation step. This is done not simply to form two magnetic layers, but the characteristics of the magnetic layers can be made preferable if the growth structure of the evaporation particles is a double structure in which two identical particle layers are formed one on the other.
When such a method is employed, however, there has been a serious problem in productivity. That is, after the first evaporation step is performed, the same evaporation step must be carried out again. In order to perform the second evaporation step, it is necessary to reset the wound original roll. Accordingly, not only does the work for resetting the roll become necessary, but a relatively long time is required after the vacuum in the vacuum vessel 32 is released until the required vacuum state in the vacuum vessel 32 can again be established.
Further, for example, when the apparatus disclosed in Japanese Patent Unexamined Publication No. Sho-56-72170, as shown in FIGS. 1 and 3, is used, the magnetic layers having the desired two-layer structure can be formed without resetting the original roll.
In the structure of the apparatus disclosed in Japanese Patent Unexamined Publication No. Sho-56-72170, however, two evaporation-material sources corresponding to the respective cooling cans are required for forming the two magnetic layers. If an evaporation source is provided for every layer to be formed, there is a problem that the apparatus becomes large in size.
Further, the evaporation stream obtained by evaporating material from the evaporation source as described above has its highest density in the vertical direction, that is, normal to the liquid surface of the fused evaporation material, and has a density distribution given by cos.sup.n .alpha. away from the normal axis. Thus, in a conventional slant incident evaporation apparatus in which the evaporation source is provided at a position offset from the central axis 26 of the cooling can 24, if it is desired to direct the evaporation stream onto the web at a certain incident angle, the central axis of the evaporation stream (the direction in which the density of the evaporation stream per unit solid angle is a maximum) will miss the web 30 to thereby deposit only a portion of the material contained in the evaporation stream on the web. Accordingly, the conventional vacuum evaporation method in which the evaporation material is deposited on the continuously conveyed web as described above has a problem in that the deposition efficiency of the evaporation material deposited on the web is extremely low. Although some solutions have been proposed, it has been impossible to expect much improvement in evaporation efficiency.
Accordingly, there have been problems that it has proven difficult to reduce material costs and to improve the production rate, especially in the case where relatively expensive nonferrous metals such as Co, Co alloy, etc. are used as the evaporation material, resulting in poor productivity.